Uninterruptible power supplies or sources (UPS) are used to provide emergency power to a load when the input power source, typically the utility mains, fails. A UPS typically provides near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry. The on-battery runtime of most uninterruptible power sources is relatively short, e.g., 5 to 15 minutes, but sufficient to allow time to bring an auxiliary power source on line, or to properly shut down the protected equipment.
FIG. 1 is a schematic block diagram of a prior art off-line UPS arrangement 100. When normal (mains) power is available, the selector 110 selects the mains power and supplies it to the load. In parallel, charger 120 is provided with power, and charges batteries 130, which are connected to an inverter 140 for converting direct current (DC) power from batteries 130 into alternating current (AC) power. When mains power is unavailable, selector 110 switches to receive power from the inverter 140, powered by batteries 130. However, the basic off-line UPS has a number of drawbacks. First, it may take a discernible time while the mains power drops below a threshold, and the selector switches to the inverter, thereby resulting in some small time of interrupted power. Therefore, in order to rectify this, the threshold may be raised; however, this may result in a greater number of switching events, some of which may not be blackouts, but small power dips or brownouts, thereby reducing battery life. An example of such a UPS arrangement may be seen in U.S. Pat. Nos. 4,823,247 and 5,081,367.
FIG. 2 is a schematic block diagram of a prior art line-interactive UPS arrangement 200, which is an improvement on the off-line UPS of FIG. 1. Selector 210, charger 220, batteries 230, and inverter 240, have similar functionality as their corresponding parts in FIG. 1, described above. In addition, the batteries 230 power an inverter 250, which contributes to the line power. Accordingly, this internal regulation on the mains path reduces battery discharge during small fluctuations in the power waveform. However, one drawback of this arrangement is that fast transients nonetheless pass through to the load. Examples of online UPS systems include U.S. Pat. Nos. 4,313,060 and 6,201,371.
FIG. 3 is a schematic block depiction of a prior art dual conversion UPS arrangement 300. In this arrangement, charger 310, batteries 320, inverter 330, and selector 340 are in the primary path, thereby providing power while the mains power is provided. Thus, the batteries are continually recharged and continually provide power to the inverter, which powers the load. The selector may bypass the batteries, for example, when mains power has been restored and the batteries are depleted. However, although the dual conversion arrangement overcomes some drawbacks of the other arrangements, it continually relies on battery power and an inverter, thereby providing power at very low efficiency. Examples of line-interactive UPS systems include U.S. Pat. Nos. 4,719,550 and 4,782,241.
Finally, FIG. 4 is a schematic block depiction of a prior art delta conversion UPS arrangement 400. Charger 410, batteries 420, and inverter 430 have similar functionality as in the previous arrangements. The internal inverter 440 provides internal regulation similar to the line-interactive arrangement of FIG. 2. In the delta conversion USP, however, the selector is replaced by a summing node 450. Therefore, rather than selecting which power source to use, which may cause detection and switching delays, the summing node provides to the load power from both sources simultaneously, drawing from the battery-powered inverter 440 power required to correct the mains power level. However, although an improvement on the other arrangements, the delta conversion UPS also suffers from disadvantages. In particular, any fast transient on the power mains will pass to the load due to the limited time response of the inverter and the architecture of the delta conversion regulation. In addition, the delta conversion will draw power from the battery-powered inverter for any voltage fluctuation, no matter how small. An example of a delta conversion UPS system includes U.S. Pat. No. 6,768,223.
There is a need for a UPS arrangement that overcomes at least some of the above drawbacks of the prior art UPS arrangements.